This invention relates to a plant control system, and more particularly to a plant control system of the so-called distributed control type in which a separate control loop is provided for each of a plurality of different control functions.
With the remarkable progress of microcomputers in recent years, the distributed-type plant control system tends to employ a microcomputer controller, in place of the conventional analog controller, in each of the control loops provided individually to different analog or digital control objects. In such a distributed-type plant control system, a plurality of controllers each including a microcomputer therein (which controllers will be referred to hereinafter as controller stations) and a display device or station for providing centralized monitoring and displaying by means such as a cathode-ray tube (CRT) are connected to one another by a transmission channel so that data transmission among the stations can be controlled.
As a known transmission control system for plant control of the distributed control type as above described, there is a so-called data highway system in which the transmission channel is provided in the form of a loop transmission line. For example, U.S. Pat. No. 4,136,384 discloses such a data highway system in which a transmission control unit is provided on the transmission channel for controlling data transmission and data is transmitted between a plurality of controller stations and a display station under control of the transmission control unit. Thus, a special-purpose transmission control unit is inevitably required in such a transmission control system. Therefore, the overall system has not the freedom of expansibility, and this leads not only to low transmission efficiency but also to low reliability.